1. Field of the Invention
The present invention relates to non-bleeding thixotropic thermally conductive materials which find special use as heat transfer materials for cooling electronic components.
2. Description of the Prior Art
U.S. Pat. No. 4,001,128 to Penneck teaches an electrical insulating material especially suitable for high-voltage applications which comprises a polymer and an anti-tracking filler system comprising at least 20% by weight of aluminum trihydrate and at least 1% by weight of an inorganic silica filler which has been treated with an organic silicone compound. While liquid silicone materials are mentioned as possible polymers, this patent fails to disclose any improvement in bleed characteristics and does not disclose the use of thermal filler powders or bleed stopping agents as contemplated in the present invention. It should be noted that aluminum trihydrate is alien to the present invention; its main function in Penneck appears to be as a water retainer.
U.S. Pat. No. 2,428,608 to Bass teaches plastic dielectric compositions which comprise a finely-divided inorganic aerogel, e.g., of silicon dioxide, aluminum oxide or iron oxide, or a mixture thereof, blended with a liquid polymer of an organo-silicone to be well adapted for sealing the space between an electric conductor and an adjacent grounded metal. This patent does not suggest the unique characteristics of the material of the present invention nor the use of a thermal filler powder having the specified characteristics called for in the present invention. It should be noted that Bass specifically suggests the use of powdered mica; mica is a good heat insulator and, since it is a mineral, its purity cannot be readily controlled. It thus cannot be substituted for either the boron or aluminum nitride of the present invention.
U.S. Pat. No. 3,882,033 to Wright teaches that organo-polysiloxane grease compositons of low noise level having, inter alia, good dielectric properties and heat transfer properties, can be obtained by utilizing certain proportioons of a polysiloxane fluid, a dielectric desiccant selected from anhydrous calcium sulfate and synthetic zeolites, and a grease thickening and thermal conducting agent. Useful thickening agents include silica, and useful thermal conducting agents include zinc oxide. This reference does not suggest the unique characteristics of the material of the present invention, nor is any specific emphasis placed upon the size of the thermal filler powder. Materials such as anhydrous calcium sulphate and synthetic zeolite are alien to the present invention; their presence would be harmful to the thermally conductive material claimed herein.
U.S. Pat. No. 2,890,170 to Ragborg teaches organo-siloxane greases having improved corrosion resistance which contain a grease thickening agent and certain proportions of benzotriazole. Useful grease thickening agents include silicon dioxide and zinc oxide.
U.S. Pat. No. 3,131,148 to Taulli teaches thickening organic liquids by the addition of amorphous metal-containing silica aerogels. The compositions are stated to be thixotropic.
U.S. Pat. No. 3,364,144 to Pulvari teaches ferroelectic, ferrielectric and pizeoeletric dielectric compositions which comprise crystalline flakes or small platelets of mixed bismuth oxides bonded with a high dielectric constant inorganic binder, for example, bismuth borate glass. Measurements by the present inventors have established that, dielectric strength is not overly influenced by particles size.
U.S. Pat. No. 3,725,275 to Carter et al. teaches that grease compositions can be thickened with aluminum carboxylate salts and, for example, silica.
U.S. Pat. No. 3,947,373 to Sobajima et al. discloses a novel electrically insulating powdery material which consists of a calcined product of a mixture of magnesium oxide and boron oxide.